The present invention relates to a multiple-wavelength optical communication system with optical amplifiers having optimized performance, and a method for optimizing the said performance.
There are known multiple-wavelength (Wavelength Division Multiplexing, WDM) optical communication systems in which the information to be transmitted is carried by a plurality of signals, each at a predetermined wavelength (channels). In long-distance WDM systems, use is increasingly made of optical amplifiers which are connected between sections of optical fibre to amplify the signals which are attenuated during propagation in the said sections, and thus to increase the distances which can be covered with these WDM systems. However, conventional optical amplifiers, for example those of the rare earth doped fibre type or the semiconductor type, have the disadvantage that they do not have a flat gain spectrum (gain as a function of the wavelength). Instead, they have a gain spectrum which varies with the wavelength according to a predetermined curve. Consequently, the different channels of a WDM system are not amplified uniformly along the transmission line, and therefore their optical signal to noise ratios (SNR) differ from each other (non-equalized optical SNR) at the receiving point at the end of the cascade of optical amplifiers.
In the present description and in the attached claims, the expression xe2x80x9coptical signal to noise ratioxe2x80x9d is used to denote the ratio, at the receiving point (at the end of the cascade of optical amplifiers), between the optical power of a channel and the optical noise power due to the spontaneous emission (Amplified Spontaneous Emission, ASE) of the optical amplifiers which is present in the optical band about the said channel. In its turn, the expression xe2x80x9coptical band of a channelxe2x80x9d is used to denote the band of the optical filter which is used in the receiving equipment of an optical communication system to filter the optical noise and to separate this channel from the others with respect to wavelength. Typically, this optical band ranges from 0.2 nm to 1 nm.
In general, the optical SNR is greater for channels having wavelengths corresponding to a higher gain, and smaller for channels having wavelengths corresponding to a lower gain. This difference in amplification and optical SNR between one channel and another increases with an increase in the number of optical amplifiers in cascade along a WDM transmission line, since the gain spectrum at the output of a chain of optical amplifiers becomes narrower and higher as the number of optical amplifiers in cascade increases (a phenomenon conventionally known as xe2x80x9cself-filteringxe2x80x9d).
Some methods have been proposed to limit the disadvantages due to the dependency of the gain of an optical amplifier on the wavelength and due to the xe2x80x9cself-filteringxe2x80x9d of an optical communication system with a chain of optical amplifiers.
Li et al. [xe2x80x9cGain equalization by mitigating self-filtering effect in a chain of cascaded EDFA""s for WDM transmissionsxe2x80x9d, Journal of Lightwave Technology, vol. 13, No. 11, pp. 2191-2196, November 1995] describe a method consisting in the use of erbium-doped optical amplifiers with alternately high and low population inversion levels along a chain of optical amplifiers.
M. A. Ali et al. [xe2x80x9cPerformance of erbium-doped fiber amplifier cascades in WDM multiple access lightwave networksxe2x80x9d, IEEE Photonics Technology Letters, vol. 6, No. 9, pp. 1142-1145, September 1994] describe a method based on the selection of parameters (e.g. pump wavelength, pump power, length of the active fibre, power of the input signals) of an erbium-doped optical amplifier.
These methods, however, have the disadvantage of requiring optical amplifiers which are suitably designed and different from one another. Consequently they are not applicable to optical communication systems already fitted with their own optical amplifiers.
K. Inoue et al. [xe2x80x9cTunable gain equalization using a Mach-Zehnder optical filter in multistage fiber amplifiersxe2x80x9d, IEEE Photonics Technology Letters, vol. 3, No. 8, pp. 718-720, August 1991] describe a filter to be connected at the output of each amplifier of a cascade of ordinary optical amplifiers to equalize its gain. However, this solution requires the installation of an optical filter for each optical amplifier and this makes the optical communication system more complex and expensive.
However, in this article no reference is made at any point to the equalization of the optical SNR.
Ashish M. Vengsarkar et al. [xe2x80x9cLong-period fiber-grating-based gain equalizersxe2x80x9d, Optics Letters, Vol. 21, No. 5, pp. 336-338, March 1996] propose that the gain of a cascade of optical amplifiers be equalized by means of a grating of optical fibres having a transmission spectrum equal to the inverse of the gain spectrum of the cascade of optical amplifiers.
However, this device is not used to equalize the optical SNR.
This disadvantage is also inherent in the device described in British Patent 2314225. This describes an optical filter for flattening the gain spectrum of an optical amplification system, comprising at least one optical amplifier, which is connected in a transmission line in which a plurality of wavelength multiplexed optical signals is transmitted. The optical filter, installed at the output of the said optical amplification system, has, like the device described by Vengsarkar, a transmission spectrum whose characteristics are the inverse of those of the gain spectrum of the said optical amplification system, and consequently the difference between the maximum loss and the minimum loss of the said transmission spectrum of the filter is equal to the difference between the maximum and the minimum of the gain spectrum of the said optical amplification system. Additionally, to overcome the fact that this device has a spectrum which varies with the temperature, the transmission spectrum of the said filter lies within a range of wavelengths which is narrower than the range of wavelengths of the gain spectrum of the said optical amplification system and is entirely contained within it, so that the temperature variations doe not shift the former range outside the latter.
Ozan K. Tonguz et al. [xe2x80x9cGain equalization of EDFA cascadesxe2x80x9d, Journal of Lightwave Technology, Vol. 15, No. 10, October 1997, pp. 1832-1841) present the results of a study which they carried out on the impact of the wavelength dependence of the gain spectrum of erbium-doped fibre amplifiers on a multiple-channel optical transmission system with direct detection.
However, these do not equalize the optical SNR and disregard the effects of non-linear phenomena which arise in an optical fibre when optical signals are transmitted with relatively high power and/or over a medium/long distance, as in submarine systems for example.
In order to equalize the optical SNR, A. R. Chraplyvy et al. [xe2x80x9cEqualization in amplified WDM lightwave transmission systemxe2x80x9d, IEEE Photonics Technology Letters, vol. 4, No. 8, pp. 920-922, August 1992] have proposed a method consisting in the transmission of the different channels of a WDM system with more or less high transmission powers according to the gain spectrum of the optical amplifiers being used. However, the inventors of the present invention have observed that this method does not take into account non-linear phenomena which arise in an optical fibre when optical signals are transmitted with relatively high power and/or over medium/long distances.
Additionally, Fabrizio Forghieri et al. [xe2x80x9cSimple model of optical amplifier chains to evaluate penalties in WDM systemsxe2x80x9d, Journal of Lightwave Technology, Vol. 16, No. 9, pp. 1570-1576, 1998] propose a mathematical model for comparing the optical SNR of a WDM optical communication system, having optical amplifiers with a non-flat spectrum, with the optical SNR of a hypothetical WDM optical communication system having ideal optical amplifiers (flat gain spectrum). More particularly, the task of this model is to identify the penalty incurred by the optical SNR of a WDM optical communication system having optical amplifiers with a non-flat spectrum when a pre-emphasis which equalizes the optical SNR is associated with the power of the optical signals, or in other words when the transmission powers of the different channels are selectively modified in such a way as to equalize the optical SNR at the reception point. In general, the aim is to attain this objective by incrementing the power of the channels having wavelengths corresponding to a lower gain as compared with the power of the channels having wavelengths corresponding to a higher gain.
However, this model also fails to take into account non-linear phenomena both in the presence and in the absence of equalizing filters of the conventional type.
Naito et al: xe2x80x9820-nm signal bandwidth after 147-amplifier chain using long-period gain-equalizersxe2x80x99 optical fiber communication conference and exhibition, OFC""98, vol. 2, 222-27 February 1998, pages 320-321, XP002121562 San Jose, Calif., USA, discloses a WDM transmission system having the feature of the preamble of claim 1. This document, however, does not appreciate that the performance of the system deteriorates because of non-linear phenomena which introduce additional noise into the channels of the system both in the presence and in the absence of conventional equalizing filters when the transmission powers of the different channels are transmitted with a pre-emphasis such that the optical SNR is equalized.
The inventors of the present invention have realized that, when the transmission powers of the different channels of a conventional WDM optical communication system are transmitted with a pre-emphasis such that the optical SNR is equalized, the performance of this system deteriorates as a result of non-linear phenomena which introduce additional noise into the different channels of the system both in the presence and in the absence of equalizing filters of the conventional type. These non-linear phenomena represent a serious problem in optical communication systems, since they arise in an optical fibre when optical signals are transmitted at relatively high powers and/or over medium/long distances and their effects increase with an increase in the power of the optical signals and in the total length of the link. Typical examples of these non-linear phenomena are four wave mixing (FWM), self phase modulation (SPM), cross phase modulation (XPM), modulation instability (MI), stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS).
In order to optimize the performance of an optically amplified WDM optical communication system while simultaneously minimizing the occurrence of non-linear phenomena, the inventors of the present invention have proposed the equalization of the optical SNR of this system with rather low levels of pre-emphasis.
This problem has been unexpectedly resolved by connecting at least one filter having a distinctive transfer function in an optically amplified WDM optical communication system.
In a first aspect, the present invention therefore relates to an optical communication system comprising
a first apparatus for supplying at least three optical signals, each having a predetermined wavelength selected from a first range of wavelengths xcex94xcex and a preselected optical power;
an optical transmission line, optically connected to the said first apparatus, for the transmission of the said at least three optical signals, the said optical transmission line comprising in its turn
i. at least one optical amplification system having a predetermined gain spectrum which, in the said first range of wavelengths xcex94xcex, has a maximum and a minimum gain (expressed in dB), the difference between the said maximum and minimum gain being xcex94Gtot, and
ii. at least one filter, associated with the said at least one optical amplification system, and capable of attenuating the power of the said at least three optical signals according to a predetermined attenuation curve as a function of the wavelength, the said attenuation curve having, in the said first range of wavelengths xcex94xcex, a maximum and a minimum attenuation (expressed in dB), the difference between the said maximum and the said minimum being xcex94L; and
a second apparatus for receiving the said at least three optical signals,
characterized in that the said power of the said at least three optical signals is associated with a preselected pre-emphasis and the said xcex94L is at least 0.5 dB greater than the said xcex94Gtot.
In the present description and in the attached claims, the expression xe2x80x9cpre-emphasisxe2x80x9d is used to denote the difference in power (Pmaxxe2x88x92Pmin) between the channel which has the highest power (Pmax) at the input of the amplifier chain and that which has the lowest power (Pmin), where the values of power are expressed in dBm.
Additionally, in the present description and in the attached claims, the values of attenuation are all expressed in absolute terms. For example, an attenuation value of 2 dB implies a decrease of 2 dB in the power of an optical signal (10 dBm, for example), so that the attenuated signal will have a power of 8 dBm.
Preferably, the said pre-emphasis of the power of the said at least three optical signals is preselected in such a way that there is an equalized optical signal-to-noise ratio at the output of the said optical transmission line.
Typically, the said pre-emphasis is at least 0.2 dB.
Preferably, the said first range of wavelengths xcex94xcex is at least 3 nm. More preferably, it is at least 5 nm. Even more preferably, it is at least 10 nm. Even more preferably, it is at least 15 nm.
Typically, the said first range of wavelengths xcex94xcex is selected from a second range of wavelengths lying between 1300 nm and 1700 nm. More typically, the said second range of wavelengths lies between 1500 nm and 1650 nm. Even more typically, it lies between 1520 nm and 1600 nm.
Typically, the said optical transmission line comprises an optical fibre incorporated, preferably, in an optical cable.
Typically, the said optical transmission line has a total length of between 300 and 9000 km. More typically, the said total length is between 300 and 6500 km. Even more typically, the said total length is between 500 and 3000 km.
Advantageously, the said at least one optical amplification system comprises at least one optical amplifier with a predetermined gain spectrum. More advantageously, the said at least one optical amplification system comprises a plurality of optical amplifiers with a predetermined gain spectrum.
Preferably, the optical amplifiers of the said at least one optical amplification system all have substantially the same gain spectrum.
Preferably, the said at least one filter is located after the said at least one optical amplification system.
When required by the path of the said optical transmission line, the said optical transmission line comprises a plurality (n) of optical amplification systems and a plurality (nxe2x88x921) of filters located between one amplification system and another.
In this case, for each filter the said difference xcex94L between the maximum and the minimum attenuation is at least 0.5 dB greater than the smallest xcex94Gtot of all the xcex94Gtot of the said (n) optical amplification systems.
In one embodiment, each of the (n) optical amplification systems comprises the same number of optical amplifiers. In an alternative embodiment, at least one optical amplification system comprises a number of optical amplifiers different from that of the other optical amplification systems.
Additionally, each of the said (n) optical amplification systems may have a xcex94Gtot different from that of the other optical amplification systems.
In one embodiment, the optical amplifiers belonging to one optical amplification system have gain spectra which are substantially the same as each other and different from those of the optical amplifiers belonging to another optical amplification system.
In another embodiment, the optical amplifiers of the said plurality of said optical amplification systems all have substantially the same gain spectrum.
Typically, the said optical amplifiers are of the optical fibre type doped with at least one rare earth. Preferably, the said at least one rare earth is erbium.
In one variant, the said optical amplifiers are of the semiconductor type.
Preferably, the said at least one filter is selected from the group of devices comprising an optical fibre grating, a micro-optical interference filter, a device formed by a combination of the two preceding technologies and an optical filter of the Mach-Zehnder type.
In a first embodiment, the said difference xcex94L between the said maximum and the said minimum attenuation is at least 0.75 dB greater than the said xcex94Gtot. In a second embodiment, the said xcex94L is at least 1 dB greater than the said xcex94Gtot. In a third embodiment, it is at least 2 dB greater. In a fourth embodiment, it is at least 3 dB greater.
Typically, the said maximum of the said attenuation curve of the said filter is located at a distance less than or equal to 5 nm from the centre of the said first range of wavelengths xcex94xcex. More typically, the said maximum is located at a distance less than or equal to 3 nm from the said centre. Even more typically, it is located at a distance less than or equal to 1 nm from the said centre. In one embodiment it is located approximately at the said centre.
Advantageously, the said minimum of the attenuation curve of the said filter has an attenuation of at least 2 dB. More advantageously, the attenuation is at least 1 dB. Even more advantageously, it is at least 0.5 dB. Additionally, the said minimum is typically located at one of the two ends of the said first range of wavelengths xcex94xcex. In one embodiment, the said attenuation curve has a minimum at both ends of the said first range of wavelengths xcex94xcex.
In a second aspect, the present invention relates to a method for optimizing the performance of a WDM optical communication system, comprising the phases of
a) supplying at least three optical signals, each having a predetermined power and having a predetermined wavelength selected from a first range of wavelengths xcex94xcex;
b) sending the said at least three optical signals along an optical transmission line;
c) amplifying, in the said optical transmission line, the said at least three optical signals according to a predetermined gain spectrum which, in the said first range of wavelengths xcex94xcex, has a maximum and a minimum gain (expressed in dB), the difference between the said maximum and the said minimum gain being xcex94Gtot;
d) attenuating the power of the said at least three optical signals, amplified in this way, according to a predetermined attenuation curve as a function of the wavelength, the said attenuation curve having, in the said first range of wavelengths xcex94xcex, a maximum and minimum attenuation (expressed in dB), the difference between the said maximum and the said minimum being xcex94L; and
e) receiving the said at least three optical signals, characterized in that the said power of the said at least three optical signals is associated with a preselected pre-emphasis and in that the said xcex94L is at least 0.5 dB greater than the said xcex94Gtot.
For information on the characteristics of the said at least three optical signals, of the said attenuation curve, of the said first range of wavelengths and of the said optical transmission line, reference should be made to what has been stated above.